The electrocardiographic diagnosis of LVH in the presence of LBBB has previously been difficult. Thirty-seven patients with complete LBBB were identified and had echocardiography performed. Using an accepted echocardiographic formula, left ventricular mass was calculated. Twenty of the 37 patients (54%) were classified as having severe LVH. Multiple conventional ECG criteria for LVH were then evaluated. No QRS voltage criteria showed any difference between patients with and without LVH (P = NS). There was also no correlation between either QRS axis or left atrial enlargement and left ventricular mass (P = NS). However, the QRS duration was significantly longer in the patients with LVH (160 +/- 12 msec) than in the normal patients (148 +/- 11 msec) (P less than 0.001). The sensitivity, specificity, positive predictive value, and accuracy of several voltage criteria and QRS duration were examined. The best voltage criteria had a sensitivity of only 50% and a predictive value of 63%. However, a QRS duration greater than 155 msec had a sensitivity of 60% and a predictive value of 82%. This study demonstrates that the conventional QRS voltage criteria for LVH are not accurate in LBBB. A relationship exists between increasing QRS duration in LBBB and LVH; therefore, the relative probability adjectives: "consider," "possible," and "probable" should be used. QRS duration greater than 155 msec is predictive of LVH despite the presence of LBBB.